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NGC 7635 - The Bubble Nebula - Emission Nebula

NGC 7635 also known as the Bubble Nebula is a HII regionemission nebula located in northwestern Cassiopeia close to the border with Cepheus. It appears round in shape due to expanding gas from the stellar wind of a massive hot central star (mag. +8.7). The nebula is located in a giant molecular cloud that glows due to excitation from the star. It was discovered by William Herschel on November 3, 1787.

NGC 7635 is located 0.5 degrees southwest of open clusterM52 with open cluster NGC 7510 positioned a further 1.5 degrees to the southwest. Although shinning at magnitude +6.9, the Bubble Nebula is a difficult object for small scopes due to its diffuse shape and low surface brightness. A 150mm (6-inch) scope is the minimum aperture recommended for this target.

The nebula is best seen from northern latitudes during the months of August, September and October when it appears high in the sky and even overhead from some locations. It's circumpolar from latitudes greater than 28N.

With a 200mm (8-inch) scope, NGC 7635 appears as a large faint shell of light that's brightest around the central star. To the north and southeast fainter wisps can be detected, which can be enhanced using an UHC filter. In addition averted vision also helps. In total, NGC 7635 spans some 15 x 8 arc minutes of apparent sky but large scopes are required to reveal significant elongation. It lies approximately 9,000 light-years distant and is number 11 in the Caldwell catalogue.